The growth and development of the oocyte in the ovulatory follicle of the rat and the interaction of the oocyte with the cells of the cumulus will be examined using quantitative cytochemistry and morphometry. The oocyte and cumulus of the ovulatory follicle develop characteristics which are distinct from those found in this complex in follicles which are undergoing atresia. In this study we will attempt to determine the earliest time in the development of the follicle in which these characteristics can be seen. Glycoprotein and mucopolysaccharide moieties in the ovum, zona pellucida and cumulus will be studied using the periodic acid-Schiff (PAS) and alcian blue reactions. Ribonucleic acid content will be measured by the gallocyanin chrome-alum procedure. Enzymes studied will include glucose-6-phosphate dehydrogenase, lactate dehydrogenase and succinate dehydrogenase. Using the localization of acid phosphatase at the ultrastructural level, the possible lysosomal origin of the cortical granules of the ovum and of PAS positive granules in the cumulus will be explored. Differences in ultrastructure between the cells of the cumulus and corona radiata in the ovulatory follicle and in follicles in early states of atresia will be quantified using morphometry. The cells of the cumulus surround the ovum, sustaining it as it grows and forming a barrier through which outside influences must pass. The eventual expulsion of a viable ovum is in part dependent on a successful symbiotic relationship between the oocyte and the cells of the cumulus and therefore a greater understanding of the way in which the cumulus and oocyte interact is crucial.